1. Field of the Invention
The present invention relates to the field of electrostatic discharging whereby electrical charges resulting from static electricity are dissipated from a charged source to an electrical ground through a resistance-controlled path. The electrical charges are dissipated in order to prevent the accidental discharge of electricity between the charged source and another object that the charged source may come in contact with. Furthermore, the resistance-controlled path protects the user or device from the passage of an externally applied high amperage current.
2. Known Art
Methods and devices for electrostatic dissipation are numerous and well known in the art, and are often referred to as ESD, or electrostatic dissipating, devices. Electrostatic dissipating is desirable in numerous applications including but not limited to footwear used while fabricating/assembling electronic circuitry, equipment used while refueling vehicles, and other uses where an accidental discharge of static electricity between the user and a device would be undesirable.
Industry standards for the dissipation of electrostatic charges are often used in the design and manufacture of known ESD devices. A known standard used to determine whether an ESD device provides safe levels of electrostatic dissipation is ANSI (American National Standards Institute) Z41, Section 6, titled xe2x80x9cStatic Dissipative Footwearxe2x80x9d. ANSI Z41 is currently undergoing revision to address the amount of variation or inconsistency in repeated resistance values of ESD footwear.
One category of known ESD devices use a grounding strap, whereby the grounding strap is physically connected from the electrostatically charged source, such as a human user or an electrical device, to an electrical ground. There exist body grounding heel straps (Van Atta et al U.S. Pat. No. 2,586,747), leg straps (Legge U.S. Pat. Nos. 2,712,098 and 2,933,651), and wrist straps (Burke U.S. Pat. No. 3,596,134) to dissipate static electricity between a human user or an electrical device and an electrical ground. The physical straps, however, are cumbersome to attach and to wear during the course of activities and severely limit the physical range of motion of the user or device requiring grounding. The use of such straps can often become such an annoyance that their use is intentionally avoided by the user, thereby resulting in increased susceptibility of electrical components to damage, and more importantly, a sacrifice in user safety. In addition, the straps are in some cases disposable, thus resulting in significant costs to a company with a considerable number of employees using and disposing the straps on a daily basis.
Another category of known ESD devices incorporate the necessary electrical elements into an otherwise single-function device such as a shoe. The use of conductive materials with a controlled ohmic path within the construction of a shoe have been provided in Edwards U.S. Pat. No. 4,785,371, and in Legge U.S. Pat. No. 2,712,099. While the known ESD footwear has the advantage of providing a self-contained capability for providing the requisite electrically resistive path for safely grounding the wearer, generally within a range of 106 to 108 ohms, the level of resistance between the wearer and the ground is unacceptably variable within this range. The level of resistance is variable in the known art because ESD properties exist in various layers throughout the shoe, (outsole, midsole, and insole), which all contribute to the resulting resistance level for the entire shoe. Edwards employs a controlled ohmic path via multiple conductive and resistive layers throughout the construction of footwear. The incorporation of the resistance throughout many layers, however, results in a wide variation in the level of total resistance provided by the shoe. Furthermore, Edwards does not provide a fail-safe mechanism within its construction in the event of a high amperage current attempting to pass through the circuit.
A critical limitation of the known art is the inability to achieve a precise and fail-safe level of resistance between the electrically charged source and the electrical ground. Precise resistance is an advantage in industrial applications where static control is critical, such as users of computer chips. Furthermore, fail-safe operation is required such that an externally applied high amperage current can be passed through the device, and more specifically through the resistance device, without resulting in harmful or even fatal results to the user and any surrounding occupants or observers.
The known ESD footwear, such as those shown in the above-mentioned Edwards and Legge patents, are incapable of providing a precise resistance level on a repeatable basis for a single ESD device in combination with a fail-safe functionality. Furthermore, there has not yet been achieved a truly reliable, fail-safe construction in which change or opening of any resistors or electrically resistive elements of the shoes will result in an acceptable degradation in overall resistance between the wearer and ground. Although multiple resistors have been proposed, as in Legge, the construction of footwear described in that patent is rendered needlessly complex because of the need to insert discrete resistors at various locations within a shoe so equipped, thus causing manufacture to be both complex and costly. Moreover, the Legge construction is not amenable to each of many styles of footwear, as to which is desired to be able to manufacture the various styles with predetermined total resistance in both normal use and fail-safe condition of use. The Legge construction incorporates xe2x80x9csafety fusesxe2x80x9d as individual parts in discrete locations of the entire ESD shoe assembly. The xe2x80x9csafety fusesxe2x80x9d, therefore, are not incorporated into a single unit that can easily be installed.
Accordingly, among the several objects, features and advantages of the invention may be noted the provision of improved devices for the dissipation of static electricity; which include a unique combination of conductive layers providing a continuous electrical path between the user or device and a ground; which dissipates static electricity through a controlled ohmic path; which repeatably and reliably delivers a precise resistance level by using at least one set of parallel resistors, whereby failure of one resistor results in a precise change in resistance level which in turn results in continued safe operation of the ESD device; which construction is adaptable to utilize standard components of a wide variety of electrical devices; which construction utilizes the standard components used in the manufacture of a wide variety of footwear styles; which construction provides a safe level of physical protection for the conductive layers and the parallel resistors; which construction utilizes only a single unit of parallel resistors within the multiple conductive layers to control the ESD properties of the entire device; which utilizes distinctive principles for both construction and operation thereby providing an ESD device capable of being adapted to a wide variety of applications.
The precise and fail-safe ESD properties are provided by a set of parallel resistors that are sandwiched between layers of conductive materials throughout the device. For example, two 3 megohm resistors connected in parallel will produce a total resistance for the device of 1.5 megohms. The new ESD footwear is intended to take advantage of a principle known in the electronics art, but not known in the art of ESD footwear, that the combined conductance of a number of parallel-connected resistors is the sum of their separate conductances:       1    R    =            1              R        1              +          1              R        2              +          1              R        3              +    ⋯  
whereby the equivalent resistance for two parallel resistors having resistances R1, R2 is:             R      =                                    R            1                    ⁢                      R            2                                    (                                    R              1                        +                          R              2                                )                      ;          xe2x80x83        ⁢                  and        ⁢                  xe2x80x83                ⁢        if        ⁢                  xe2x80x83                ⁢        R1            =      R2        ,            ∴      R        =          R      /      2      
Therefore, with two resistors each having a resistance of 3 megohms, the total resistance for the circuit would be one half of the resistance value of the common resistance value, or 1.5 megohms. Furthermore, if one resistor should fail, the resistance of the circuit becomes 3 megohms, well within safe operating limits for typical ESD applications, thereby providing continued safe operation of the ESD device. Depending on the level of resistance required for the ESD application, the value of each parallel resistor can be adjusted accordingly.
The construction of the conductive layers and the parallel resistors within the layers is such that the apparatus repeatably delivers a precise resistance value. As will become apparent in the forthcoming detailed description, the distinct and unique construction employed to achieve accurate and fail-safe resistance can be incorporated into a multitude of ESD applications.
Briefly, an electrostatic dissipating device of the present invention, as in the form of new ESD footwear, provides both controlled dissipation of electrostatic charges and fail-safe operation through a unique construction of conductive layers and at least one set of parallel resistors. The conductive layers are achieved through the use of a conductive thread sewn to interior footwear layers wherein wire leads of the set of parallel resistors contact the conductive thread on one side, and subsequently contact conductive members of an outer sole on the other side, thereby comprising the distinctive conductive and resistive layering that subsequently grounds the user or device to the ground.
Other objects and features will be apparent or are pointed out more particular herein below.